Heat resistant alloy



III

Patented Feb. 12,1935

UNITED STATES- HEAT RESISTANT ALLOY Hana ium.-Milwaukee, Wla, assignor to A. 0.

Smith Corporation, Milwaukee, Wis., a corporation of New York- No Drawing.

Application June 25, 1932,

Serial No. 619,358

MOIaims.

The invention relates to alloy steels which contain relatively large amounts of aluminum and it is among the objects of the invention to provide steels which have a high ohmic resistance and great resistance to deterioration or scaling especially at high temperatures, and to provide resistant steels of the kind so that they can be practicabLv worked or forged into shapes, sheets, wires or ribbons. Another object of the invention is to provide resistance elements composed of these alloys which are suitable and durable" when they are used at moderately high temperatures.

In accordance with the invention, the alloys may be considered generally as high aluminum low carbon-chromium steels which contain small amounts of other elements that give the steels properties suiting them for use as heating ele-v ments and for manufacturing purposes. The alloys contain small amounts of silicon, from about 5% to 10% chromiumand about 16% to 20% aluminum. The alloys are heatresistant and are especially suitable for making electrical resistance elements when they contain suitable amounts of modifying elements because of their durability and high ohmic resistance.

It was found that steels of this aluminum content are not workable into the desired shapes for heating elements if they'contain more than about 0.05% carbon and that more than this amount of carbon is prohibitive in manufacturing attenuated articles from the alloys which are to be used for heating elements. Although carbon is commonly present in the alloys, they may be free from carbon. For the best results, the carbon should be as low as possible and not over 0.05% carbon. q

The alloys of iron, chromium and aluminum-are not well adapted for fabricating attenuated articles unless they are modified by small additions of other elements which will be hereinafter described. Without additional or modifylngelements, the forge'abilityof alloys which contain even-less than 16% aluminum is not satisfactory for manufacturing purposes while those which contain more than l6% are practically-unforgeable without the additions. I With more than about 20% aluminum, the workability and resistance to oxidation at high temperatures are not good in that the alloys lack cohesion at the grain boundaries. About 5% to 10% chromium improves the workability of the alloys, but in those which contain more than about 815% chromium and 16% of aluminum, the chromium is not efl'ective and larger amounts of chromium lower the ohmic resistance and the resistance to scaling.

The high aluminum alloys machine poorly if they contain more than about-8% chromium.

The best' results are obtained when the alloy is carefully deoxidized. Silicon is commonly present either as an impurity in the'materials used for making the alloys or as a residue of the deoxidizer. Fractional percentages of "silicon are desirable in that they improve the resistance to scaling but not more than about 0.25% may be present. Larger amounts of silicon decrease the forgeability.

Even smaller amounts of silicon than 0.25% decrease the forgeability unless one or more effective grain refining elements are added. I Titanium and molybdenum have been found to be effective grain refining elements. About 0.1% to 0.5% of the grain refiner is used. Molybdenum can be used for this purpose instead of titanium, but it may desirably be present in addition to titanium since it strengthens the grain refining action of titanium. Increased amounts oLmolybdenum up to about 1.5% are beneficial in that these amounts increase the ohmic resistance. The highest resistances can desirably be attained by additions of 1.0% to 1.5% molybdenum, but these alloys are suitable for use only at temperatures below about 1200 C. sinceloss of molybdenum is noticeable at higher temperatures, such as 1400 C. Molybdenum in amounts of 0.5% to 1.5% in conjunction with manganese in amounts of about 0.5% to 1.5% gives the alloys practically a zero temperature coeflicient of resistance. From considerations of scaling, forgeability, ohmic resistance and temperature coeflicient of resistance, thecombination of about 0.5% to 1.0% manganeseand 0.5% to 1.0% molybdenum with orwithout titanium may be mentioned as a desirable range.

I have found that alloy'heating elements containing more than 16% to about 20% aluminum, about 5% to 8.5% chromium, about 0.1% to 0.5% titanium-and about 0.1% to 1.0% molybdenum give excellent workability and resistance to scaling. In the alloys containing this aluminum content the titanium is essential for ready workability and the molybdenum adapts the alloys to practicable shop conditions. Materially more than 0.5% of titanium decreases the ohmic resistance, and more than 1 .5% of molybdenum makes the alloys non-workable and non-machinable.

An addition of about 0.4% to 1.5% of manganese will give a practically zero temperature coefllcient of resistance. Larger amounts of manganese lower the resistance to scaling. The best results are not obtained when the highest amounts of both chromium and manganese are present. Therefore, smaller amounts of manganese may be used with the highest chromium contents and smaller amounts of chromium may be used when higher contents of manganese are present.

'A representative ingot containing 17.5% aluminum, 815% chromium, 0.44% manganese, 0.36% titanium, 0.02% carbon, 0.13% silicon and the remainder iron with small fractional amounts of sulphur and phosphorus was found to have excellent forging properties when the casting was made in a mold, such as a sand mold, in which chilling was avoided. Ribbon was made-from the ingot by hot forging and rolling. These ribbons were used for heating elementsby passing current therethrough. The alloy had practically a zero temperature coeill'cient. of resistance.

The resistance of the elements was about 220 microhms per cubic centimeter at 25 C. and they showed excellent durability and freedom from scaling when they were heated to 1200 C. for long. periods of time. Other alloys, which were similar except that in addition to titanium, they contained molybdemunfrom 0.5% to about 1.0%, were easier to work when cast under'the same conditions and they had excellent forging properties when some chilling was present in the mold. An increase of carbon to 0.25% or silicon to 0.5% made the alloys unforgeable. An increase of the chromium to 9.5% made the alloy unforgeable or so difficult to forge that it was impracticable to apply it to the manufacture of attenuated heating elements. a

High ohmic resistances are found in the alloys containing more than 16% up to 18% aluminum. namely 200 to 240 microhms per centimeter cube.

This resistance is about twice that of alloys contalning approximately 80% nickel and 20% chromium. Comparative tests with the chromium-nickel heating elements showed that the above described heating elements lost less than half as much by scaling when they were heated to temperatures of about 1400 C. in air. The chromium-nickel elements became pitted and dull in appearance whereas the described elements remained smooth and retained a light gray color.

Although the limiting proportions of the various constituents are specifically recited, it is to be understood that various changes, even asgreat as several tenths of one percent in the largest components may be made without departing from thesinvention.

I claim:

1. A heat resistant alloy composed of more than 16% to about 20% aluminum. about 5% to 8.5% chromium, about 0.4% to 1.5% manganese, small mounts of silicon but not more than about 0.25% silicon, andabout 0.1% to 0.5% titanium.

the balance being substantially all iron.

2. A heat resistant alloy composed of more than'16% to about 20% aluminum, about 5% to 8.5% chromium, about 0.4% to 1.5% manganese. small amounts of siliconbut not more than about 0.25% silicon, and about 0.1% to'1.5% molybdenum, the balance being substantially all iron.

3. A heat resistant alloy composed of more than 16% to about 20% aluminum, about 5% to 8.5% chromium, about 024% to 1.5% n small amounts of silicon but not more than-about 0.25% silicon, about 0.1% to 0.5% of at least one of the'metals of the group consisting of titanium and molybdenum, the balance being substantially all iron.

4. A heat resistant alloy composed of more than 16% to about 20% aluminum, about 5% toresistance of the alloy, the balance being sub-,

stantiallyall iron.

6. A heat resistant alloy composed of more than 16% to about 20% aluminum, about 5% to 8.5% chromium, about 0.4% to 1.0% manganese, small amounts of silicon but not more than about 0.25%, about 0.1% to 0.5% of at leastone of the group of elements which consists of titanium and molybdenum, and an additional amount of molybdenum not exceeding a total 'of about 1.0%, the balance being substantially all iron.

7. A heat resistant alloy composed of more than 16% to about 18% aluminum, about 5% to 8.5% chromium, about 0.4% to 1.0% manganese,

about 0.1% to 0.5% titanium, about 0.1% to 1.0% molybdenum, and small amounts of .carbon and silicon but not more than about 0.05% car- -bon and 0.25% silicon, the balance being substantially all iron.

8. Heating element consisting of an attenuated body of alloy which is composed of more than 16% to about 20% aluminum,about 5.0% to 8.5% chromium, about 0.4% to 1.5% manganese, small amounts of silicon but not more than 0.25%. and about 0.1% tot0.5,% of atleast one of the metals of the group which consists of titanium and molybdenum, ,the balance being substantially all iron.

9. Heating element consisting of an attenuated body of alloy which is composed of more than 18% to about 20% aluminum, about.5.0% to 8.5% chromium, about 0.4% to 1.5% manganese, small amounts of silicon but, not more than 0.25%, about 0.1% to 0.5% of at least oneof the group whichconsists of titanium and molvbdenum, and an additional amount of molybdenum not exceeding a total of about'"1.0%, the

balance being substantially all iron;

10. Heating element ,consisting'of an attenuated body of alloy which is composed-of more,

than 16% to about 20% aluminum, about 5.0% to 8.5% chromi about 0.4% to 1.5% manganese, small am unts of silicon but not more than 0.25%, about 0.1% to 0.5% titanium, and about 0.1% to 1.5% molybdenum, the balance being substantially all iron.

ILHeating element consisting of an attenuated body of alloy which is composed of more than 16% to about 20% aluminum, about 5.0% to 8.5% chromium, about 0.4% to 1.5% manganese, small amounts of silicon but not more than 0.25%, about 0.1% to 015% titanium, and

about 0.1% to 1.0%: molybdenum, the balance being substantially all iron.

' 12. A heat resistant alloy composed of more thaif 16% to about 20% aluminum, 5% to 8.5% chromium, 0.4% to 1.5% manganese, small amounts of silicon but not more than 0.25%, and a minimum of 0.1% of'at least one of the group of elements consisting of titanium and molybdenum, the titanium not exceeding" about 0.5%

and the molybdenum not exceeding about 1.5%. the balance being substantially all iron.

18. An alloy for electric resistance elements having a speciilc resistance of at least 200 microhms per centimeter cube. composed of about 16% to 18% aluminum. 5% to 10% chromium. 0.4% to 1.6% manganese. small amounts of carbon and silicon but not more than 0.05% carbon and 0.25% silicon, and about 0.1% to 0.5%

titanium, the balance being substantially all iron.

14. An alloy for electric resistance elements having a speciiic resistance oi at least 200 microhms per centimeter cube and a temperature coeflicient of resistance approximately equal to zero, composed 01' about 17.5% aluminum, about 8.5% chromium, about 0.4% manganese. about 0.4% titanium. and silicon but less than 0.25%, the balance being substantially all iron. 

